Process for secondary recovery

ABSTRACT

Hydrocarbons are recovered from a subterranean hydrocarbon-bearing formation penetrated by an injection well and a production well by displacing hydrocarbons toward the production well using a drive fluid such as water thickened with polyacrylamide or partially hydrolyzed polyacrylamide or the sodium, potassium or ammonium salt thereof and a minor amount of polyacrylamide or partially hydrolyzed polyacrylamide or the sodium, potassium or ammonium salt thereof alkoxylated with an alkylene oxide. Optionally, the drive fluid can be saturated with carbon dioxide and/or natural gas at the injection pressure. An aqueous fracturing fluid containing a small amount of alkoxylated polyacrylamide or partially hydrolyzed polyacrylamide is also described.

This is a continuation-in-part of application Ser. No. 953,375, filedOct. 23, 1978, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for recovering hydrocarbons from asubterranean hydrocarbon-bearing formation penetrated by an injectionwell and a production well wherein a drive fluid such as water thickenedwith (I) polyacrylamide or partially hydrolyzed polyacrylamide or thesodium, potassium or ammonium salt thereof and (II) a minor amount ofpolyacrylamide or partially hydrolyzed polyacrylamide or the sodium,potassium ammonium salt thereof alkoxylated with ethylene oxide or amixture of ethylene oxide and propylene oxide is utilized to displacehydrocarbons in the formation toward a production well. In anotheraspect this invention relates to a method of fracturing a formationusing an aqueous fluid containing a small amount of alkoxylatedpolyacrylamide or partially hydrolyzed polyacrylamide.

2. Prior Art

The production of petroleum products is usually accomplished by drillinginto a hydrocarbon-bearing formation and utilizing one of the well-knownrecovery methods for the recovery of hydrocarbons. However, it isrecognized that these primary recovery techniques may recover only aminor portion of the petroleum products present in the formationparticularly when applied to reservoirs of viscous crudes. Even the useof improved recovery practices involving heating, miscible flooding,water flooding and steam processing may still leave up to 70-80 percentof the original hydrocarbons in place.

Thus, many large reserves of petroleum fluids from which only smallrecoveries have been realized by present commercial recovery methods,are yet to reach a potential recovery approaching their estimatedoil-in-place.

Water flooding is one of the more widely practiced secondary recoverymethods. A successful water flood may result in recovery of 30-50percent of the original hydrocarbons left in place. However, generallythe application of water flooding to many crudes results in much lowerrecoveries.

The newer development in recovery methods for heavy crudes is the use ofsteam injection which has been applied in several modifications,including the "push-pull" technique and through-put methods, and hasresulted in significant recoveries in some areas. Crude recovery of thisprocess is enhanced through the beneficial effects of the drasticviscosity reduction that accompanies an increase in temperature. Thisreduction in viscosity facilitates the production of hydrocarbons sinceit improves their mobility, i.e., it increases their ability to flow.

However, the application of these secondary recovery techniques todepleted formations may leave major quantities of oil-in-place, sincethe crude is tightly bound to the sand particles of the formation; thatis, the sorptive capacity of the sand for the crude is great. Inaddition, interfacial tension between the immiscible phases results inentrapping crude in the pores, thereby reducing recovery. Anotherdisadvantage is the tendency of the aqueous drive fluid to finger, sinceits viscosity is considerably less than that of the crude, therebyreducing the efficiency of the processes. Another disadvantage is thetendency of the aqueous drive fluid to remove additional gas bydiffusion from the in-place oil, thus further reducing the alreadylowered formation oil volume and increasing the viscosity of the oil.

There is a definite need in the art for a water flooding process inwhich the disadvantages discussed above are largely eliminated oravoided.

SUMMARY OF THE INVENTION

This invention relates to a process for recovering hydrocarbons from asubterranean hydrocarbon-bearing formation penetrated by an injectionwell and a production well which comprises:

(A) injecting into the formation via an injection well a drive fluidcomprising water having dissolved therein about 0.05 to about 5 weightpercent of polyacrylamide or partially hydrolyzed polyacrylamide or thesodium, potassium or ammonium salt thereof and a minor amount ofpolyacrylamide or partially hydrolyzed polyacrylamide or the sodium,potassium or ammonium salt thereof alkoxylated with a material selectedfrom the group consisting of (a) ethylene oxide or (b) a mixture ofethylene oxide and propylene oxide wherein the weight percent ofethylene oxide in the said mixture is from about 60 to about 95,

(B) forcing the said fluid through the formation and

(C) recovering hydrocarbons through the production well.

An additional embodiment of this invention relates to the driving fluidcompositions utilized in step (A) which may be saturated with carbondioxide and/or natural gas, if desired.

DETAILED DESCRIPTION OF THE INVENTION

Prior to practicing the process of this invention it is sometimesdesirable to open up a communication path forcing a liquid such aswater, oil or any other suitable hydrocarbon fraction into the formationat pressures of from about 300 to about 3000 psig which are sufficientto rupture the formation and to open up channels therein. By use of thismethod it is possible to position the fracture at any desired verticallocation with respect to the bottom of the oil-filled zone. It is notessential that the fracture planes be horizontally oriented, although itis, of course, preferable that they be so oriented. After the fracturehas been established, and without diminishing the fracture pressure, apropping agent may be injected into the fraction in order to preventhealing of the fracture which would destroy its usefulness for fluidflow communication purposes. Gravel, metal shot, glass beads, sand, etc.and mixtures thereof are generally employed as propping agents. Whensand is utilized as the propping agent particles having a Tyler meshsize of from about 8 to about 40 are preferred (i.e., from about 0.016to about 0.093 inches).

Generally, the number average molecular weight of the polyacrylamide orpartially hydrolyzed polyacrylamide or salts thereof utilized in thisinvention and of the alkoxylated polyacrylamide or partially hydrolyzedpolyacrylamide or salts thereof will range from about 10,000 to about2,000,000 or more. Polyacrylamide, partially hydrolyzed polyacrylamideor salts thereof which are manufactured and sold by a number of chemicalmanufacturers, are prepared by the usual vinyl compound polymerizationmethods.

The partially hydrolyzed polyacrylamides which are useful in preparingthe ethoxylated, partially hydrolyzed polyacrylamides employed in thedrive fluid of this invention consist of about 12 to about 67 molepercent of recurring units of the formulas: ##STR1## where M representshydrogen, sodium, potassium or ammonium and about 33 to 88 mole percentof recurring units of the formula: ##STR2## The preparation of suchpartially hydrolyzed polyacrylamides is well known in the art and isdescribed in detail in U.S. Pat. Nos. 3,039,529; 3,002,960; 3,804,173,etc.

The alkoxylated polymers employed in the process of this inventioncomprise (1) polyacrylamide or (2) partially hydrolyzed polyacrylamideor the sodium, potassium or ammonium salt thereof alkoxylated with,i.e., reacted with, from about 2 to about 100 percent by weight ofethylene oxide or with a mixture of ethylene oxide and propylene oxidewherein the weight percent of ethylene oxide in the said mixture isabout 60 to about 95. In another embodiment, alkoxylated polymers usefulin the secondary recovery process of this invention are prepared byreacting polyacrylamide or partially hydrolyzed polyacrylamide with2,3-epoxy-1-propanol (i.e., glycidol).

The alkoxylation of the acrylamide polymers, i.e., the reaction of thesepolymers with an alkylene oxide, can be conveniently conducted usingmethods well known in the art. For example, an aqueous solutioncomprising about 10 to about 30 weight percent or more of the acrylamidepolymer in water along with about 0.5 weight percent or more ofpotassium hydroxide or sodium hydroxide is charged to an autoclave andthe autoclave and contents heated to a temperature of about 125° toabout 200° C. after which the required weight of ethylene oxide or amixture of ethylene oxide and propylene oxide is pressured with nitrogeninto the autoclave over a period of 1 to about 3 hours or more followingwhich the autoclave is allowed to cool to room temperature and thenvented. The reaction product remaining after being stripped to removevolatile materials yields the water-soluble, alkoxylated polymer.

A number of other methods are set out in the art for conducting suchalkoxylation reactions including those described in U.S. Pat. Nos.2,213,477; 2,233,381; 2,131,142; 2,808,397; 3,879,475; 2,174,761;2,425,845; 3,062,747; 3,380,765 and German Offen. No. 2,021,066 of Nov.11, 1971 (CA 76 86780b (1972)).

The following example which illustrates the preparation of thealkoxylated acrylamide polymers is to be considered not limitative.

EXAMPLE I

A total of 450 cc of water, 5 g. of powdered potassium hydroxide and 65g. of polyacrylamide (number average molecular weight of about 250,000)are added to an autoclave which is then heated to a temperature of 125°C. Ethylene oxide in the amount of 40 g is added to the autoclave undernitrogen pressure over a 1.05 hour period during which time thetemperature of the autoclave is maintained at 130° C. Next, theautoclave and contents are allowed to cool to room temperature afterwhich the autoclave is vented. The reaction mixture is then stripped ofvolatiles using a nitrogen purge. The resulting water-soluble product ispolyacrylamide alkoxylated with about 37 weight percent of ethyleneoxide.

In the secondary recovery process of this invention, generally theaqueous drive fluid will contain from about 0.05 to about 5 weightpercent or more of the polyacrylamide or partially hydrolyzedpolyacrylamide or the sodium, potassium or ammonium salt thereof andfrom about 0.01 to about 2.0 weight percent of the alkoxylatedpolyacrylamide or partially hydrolyzed polyacrylamide or salts thereof.Optionally,, the aqueous drive fluid may be saturated with carbondioxide and/or natural gas at the injection pressure which generallywill be from about 300 to about 3000 psig or more.

If desired, the aqueous drive fluid having dissolved therein theabove-described polymeric thickening agent may be made alkaline byaddition of an alkaline agent. The advantageous results achieved withthe aqueous alkaline medium used in the process of this invention arebelieved to be derived from the wettability improving characteristics ofthe alkaline agent.

Useful alkaline agents include compounds selected from the groupconsisting of alkali metal hydroxides, alkaline earth metal hydroxides,and the basic salts of the alkali metal or alkaline earth metals whichare capable of hydrolyzing in an aqueous medium to give an alkalinesolution. The concentration of the alkaline agent employed in the drivefluid is generally from about 0.005 to about 0.3 weight percent. Also,alkaline materials such as sodium hypochlorite are highly effective asalkaline agents. Examples of these especially useful alkaline agentsinclude sodium hydroxide, potassium hydroxide, lithium hydroxide,ammonium hydroxide, sodium hypochlorite, potassium hypochlorite, sodiumcarbonate and potassium carbonate.

A wide variety of surfactants such as linear alkylaryl sulfonates, alkylpolyethoxylated sulfates, etc. may also be included as a part of theaqueous drive fluid composition. Generally about 0.001 to about 1.0 ormore weight percent of the surfactant will be included in the drivefluid.

This invention is best understood by reference to the following examplewhich is offered only as an illustrative embodiment of this inventionand is not intended to be limitative.

EXAMPLE II

In a field in which the primary production has already been exhausted,an injection well is completed in the hydrocarbon-bearing informationand perforations are formed between the interval of 3970-3985 feet. Aproduction well is drilled approximately 410 feet distance from theinjection well, and perforations are similarly made in the samehydrocarbon-bearing formation at 3975-3990 feet.

The hydrocarbon-bearing formation in both the injection well and theproduction well is hydraulically fractured using conventionaltechniques, and a gravel-sand mixture is injected into the fracture tohold it open and prevent healing of the fracture.

In the next step water saturated with carbon dioxide at 1200 psig at atemperature of 70° F. to which there has been added about 0.22 weightpercent sodium hydroxide and containing dissolved therein 0.32 weightpercent of a polyacrylamide having a number average molecular weight ofabout 135,000 and 0.12 weight percent of the sodium salt of partiallyhydrolyzed polyacrylamide having about 16 mole percent of thecarbonamide groups originally present in the polyacrylamide hydrolyzedto carboxyl groups and having a number average molecular weight of300,000 alkoxylated with about 13 weight percent of ethylene oxide inthe manner previously described in Example I above is injected via theinjection well into the formation at a pressure of about 1200 psig andat the rate of 0.95 barrel per minute. Injection of the driving fluid iscontinued at the rate of about 1 barrel per minute and at the end of 63days the rate of production of oil is substantially greater than withwater injection alone.

Another embodiment of this invention relates to a method of fracturing afluid-bearing formation such as a hydrocarbon-bearing formation wherebyartificial channels or fractures of high fluid conductivity within theformation can be formed. The pressures employed in the fracturingprocess may range from 500 to 10,000 psi or more (measured at thesurface).

The method of this invention for fracturing a fluid-bearing formationpenetrated by a well comprises injecting a viscous fracturing fluid downthe well and into contact with the said fluid-bearing formation at apressure and volume flow rate sufficient to fracture the said formationand wherein the viscous fracturing fluid comprises an aqueous solutionof a water-soluble, high molecular weight, partially hydrolyzedpolyacrylamide alkoxylated with an alkylene oxide and, optionally, asmall amount of amorphous colloidal silica.

The novel viscous fracturing fluid comprises an aqueous base, which ispreferably water, containing from about 0.01 to about 1.0 weight percentof a water-soluble, partially hydrolyzed polyacrylamide alkoxylated inthe manner previously described above with about 2 to 100 weight percentor more of ethylene oxide or a mixture of ethylene oxide and propyleneoxide wherein in the said mixture the weight percent of ethylene oxideis about 65 to about 95. If desired, the fracturing fluid may includesuspended therein about 0.2 to about 4.8 pounds per gallon of a proppingagent such as sand grains having a Tyler mesh size of about 8 to about40. Further, the fracturing fluid may contain suspended therein about0.05 to about 0.5 weight percent of amorphous colloidal silica having aparticle size range of from about 7 to 15 millimicrons such as that soldby the Cabot Corp. of Boston, Mass. under the tradename CAB-O-SIL®.Surfactants such as ethoxylated nonyl phenol, etc. can also be added tothe fracturing fluid to increase the viscosity.

The number average molecular weight of the partially hydrolyzedpolyacrylamides useful in preparing the alkoxylated polyacrylamidesemployed in the fracturing fluids of this invention will range fromabout 500,000 to about 2,000,000 or more. The partially hydrolyzedpolyacrylamides utilized in forming the alkoxylated materials will havefrom about 12 to 67 mole percent of the original carboxamide groupshydrolyzed to carboxyl groups.

The fracturing method of this invention is illustrated by the followingexample which is to be considered not limitative.

EXAMPLE III

An oil bearing formation at 3440-3505 ft. composed primarily oflimestone having good porosity (about 23%) contains a large quantity ofoil (about 62% of the pore space) and adequate reservoir pressure (about650 psi) but has a very low permeability (less than 1.1 millidarcies)and the productivity is about 4 bbls. oil per day.

The formation is fractured in 4 stages using conventional techniques forproportioning the fracture fluid into each set of perforations. Thefracture fluid is water having 0.2 weight percent of partiallyhydrolyzed polyacrylamide having about 16 mole percent of the originallypresent carboxamide groups hydrolyzed to carboxyl groups and having anumber average molecular weight of about 655,000 which provides aviscosity sufficient to permit a sand concentration of 3 lb./gal.

The formation is fractured using this fluid and sand (100,000 lbs.total). The sand size is:

    ______________________________________                                        First 80,000 lb.      20-40 mesh                                              Remaining 20,000 lb.  10-12 mesh                                              ______________________________________                                    

The well is placed on production and produces at a rate substantiallyabove that experienced before the fracturing step was completed.

A substantial reduction in the friction loss in the fracturing processof this invention can be achieved when the proppants, such as sandgrains, employed in the viscous aqueous fracturing fluid are coatedwith, for example, a thin film of polyacrylamide or partially hydrolyzedpolyacrylamide in which about 12 to about 67 mole percent of theoriginal carboxamide groups have been hydrolyzed to carboxyl groups,propoxylated with about 20 to about 100 weight percent of propyleneoxide or a mixture of propylene oxide and ethylene oxide where theweight percent of propylene oxide in the mixture ranges from about 65 toabout 95. The propoxylated polyacrylamides which are water insoluble canbe prepared by methods well known in the art by reacting, for example, apropylene glycol of suitable molecular weight with the polyacrylamide inthe presence of sodium hydroxide at a temperature of about 100° C. Thenumber average molecular weight of the polyacrylamide or partiallyhydrolyzed polyacrylamide suitable for use in making the propoxylatedmaterials will vary from about 200,000 to about 2,000,000 or more.

The thickness of the propoxylated polymer coating on the proppant neededwill range from about 5 microns up to a preferred thickness about 0.001inch to about 0.005 inch although a thickness more or less than set outin this range can be utilized, if desired.

In preparing polymer-coated, sand grains a solution of from 0.01 toabout 1.5 percent or more by weight of the propoxylated polymer is firstprepared by dissolving the polymer in ethyl alcohol, acetone or anyother suitable solvent, at temperatures about 50° to about 100° C. Sandgrains having a Tyler mesh size of about 8 to 40 are then added withstirring to the thus-prepared polymer solution after which thepolymer-coated grains are separated from the solution and dried. In aspecific illustration of the preparation of such polymer-coated sandgrains a solution of 0.03 weight percent is formed by adding partiallyhydrolyzed polyacrylamide having about 16 mole percent of thecarboxamide groups hydrolyzed to carboxyl groups alkoxylated with 32weight percent of propylene oxide (number average molecular weight650,000) with stirring to 1,000 gallons of ethyl alcohol at 70° C. Atotal of 2,000 lbs. of clean sand having a Tyler mesh size of 20 isadded with vigorous mixing to the alcohol solution maintained at about105° C. Mixing is continued for about 25 minutes after which the coatedsand grains are recovered by filtration and then dried by tumbling inhot air at a temperature of about 160° F.

What is claimed is:
 1. A process for recovering hydrocarbons from asubterranean hydrocarbon-bearing formation penetrated by an injectionwell and a production well which comprises;(a) injecting into theformation via an injection well a drive fluid comprising water havingdissolved therein about 0.05 to about 5 weight percent of a material ofnumber average molecular weight from about 10,000 to about 2,000,000selected from the group consisting of (I) polyacrylamide and (II)partially hydrolyzed polyacrylamide or the sodium, potassium or ammoniumsalt thereof and from about 0.01 to about 2.0 weight percent of asubstance selected from the group consisting of (III) polyacrylamide and(IV) partially hydrolyzed polyacrylamide or the sodium, potassium orammonium salt thereof alkoxylated with about 2 to about 100 weightpercent of 2,3-epoxy-1-propanol wherein in the partially hydrolyzedpolyacrylamide or the sodium, potassium or ammonium salts thereof theproportion of amide containing monomers is about 33 to about 88 molepercent and the proportion of the carboxy containing monomers is about12 to about 67 mole percent, (b) forcing the said fluid through theformation, and (c) recovering hydrocarbons through the production well.2. The process of claim 1 wherein the said drive fluid contains asurfactant.
 3. The process of claim 1 wherein the said drive fluid issaturated with carbon dioxide at a pressure of about 300 to about 3000psig.
 4. The process of claim 1 wherein the said drive fluid issaturated with natural gas at a pressure of about 300 to about 3000psig.
 5. The process of claim 1 wherein the number average molecularweight of the said alkoxylated substance is from about 10,000 to about2,000,000.
 6. The process of claim 1 wherein the said alkoxylatedsubstance is alkoxylated, partially hydrolyzed polyacrylamide or thesodium, potassium or ammonium salt thereof.
 7. The process of claim 1wherein the said drive fluid includes about 0.005 to 0.3 weight percentof an alkaline agent selected from the group consisting of sodiumhydroxide, potassium hydroxide and sodium hypochlorite.
 8. A fluid foruse as a drive fluid in recovering hydrocarbons from subterraneanformations comprising water having dissolved therein about 0.05 to about5 weight percent of a material of number average molecular weight fromabout 10,000 to about 2,000,000 selected from the group consisting of(I) polyacrylamide and (II) partially hydrolyzed polyacrylamide or thesodium, potassium or ammonium salt thereof and 0.01 to about 2.0 weightpercent of a substance selected from the group consisting of (III)polyacrylamide and (IV) partially hydrolyzed polyacrylamide or thesodium, potassium or ammonium salt thereof alkoxylated with about 2 toabout 100 weight percent of 2,3-expoxy-1-propanol, wherein in thepartially hydrolyzed polyacrylamide or the sodium, potassium or ammoniumsalts thereof the proportion of amide containing monomers is about 33 toabout 88 mole percent and the proportion of the carboxy containingmonomers is about 12 to about 67 mole percent.
 9. The fluid of claim 8saturated with carbon dioxide at a pressure of about 300 to about 3000psig.
 10. The fluid of claim 8 saturated with natural gas at a pressureof about 300 to about 3000 psig.
 11. The fluid of claim 8 wherein about0.005 to about 0.3 weight percent of an alkaline agent selected from thegroup consisting of sodium hydroxide, potassium hydroxide and sodiumhypochlorite is included.
 12. The fluid of claim 8 wherein about 0.001to about 1.0 weight percent of a surfactant selected from the groupconsisting of alkylaryl sulfonates and alkyl polyethoxylated sulfates isincluded.